Process for the manufacture of metallic niobium or tantalum or alloys thereof



Nov. 12, 1963 w. SCHELLER ETAL 3,110,585 PROCESS FOR THE MANUFACTURE OF METALLIC momma 0R TANTALUM 0R ALLOYS THERFOF Filed Oct. 20. 1960 I8 16 FF INVENTORS Walter Small Max B/umer by LAMA-M, M W

' ATTORNEYS United States Patent 3,116,585 PROCESS FOR THE MANUFACTURE OF METAL- LIC NIOBKUM 0R TANTALUM 0R ALLOYS THEREOF Walter Scheller, Neneweit, Switzerland, and Max Blnmer, Woods Hole, Mass, assignors to Ciba Limited, Basel, Switzerland, a company of Switzerland Filed Oct. 20, 1960, Ser. No. 63,899 Claims priority, application Switzerland Oct. 21, 1959 1 Claim. (Cl. 7584.5)

This invention relates to the manufacture of metallic niobium or tantalum or an alloy :of these two metals by the reduction of their halides with magnesium.

It is known that the high melting metals niobium and tantalum can be obtained by reducing their halides with magnesium. The known processes have the disadvantage that the halides generally used, especially the pentachlorides, are difiicult to treat owing to their high vapor pressure and their sensitivity to air and moisture, so that complicated apparatus is required for carrying out the reduction. The high sensitivity of the pentahalides to moisture is especially disadvantageous. Thus, even when a very small amount of moisture gains access to the pentahalides before the reduction, the oxyhalides particularly are formed and this leads to an undesired high content of oxygen in the metal.

The present invention provides a process for the manutacture 0t metallic niobium or tantalum or an alloy of these two metals by the reduction of a halide of the metal with magnesium, wherein a starting material comprising a niobium halide and/ or tantalum halide of lowvapor pressure is or are mixed in a finely divided form with finely divided magnesium, the mixture is ignite-d by locally heating it, and, after the reaction, the metal or metals is or are separated from the other reaction products.

There are advantageously used as halides having a low vapor pressure halogen compounds or alkali metal halide double salts of niobium and/or tantalum of low valency.

The halides of niobium or tantalum of low valency, especially niobium trichloride, can be obtained, for example by reducing the pentachloride or chlorides with hydrogen. The pentaohlorides can be obtained in known manner by the chlorination of oxidic compounds of the metals, especially oxidic ores thereof, with chlorine in the presence Off a reducing agent. Processes of this kind are described, for example, in US. Patent No. 2,429,671 granted October 28, 1947, to Cuvelliez.

Alkali metal halide double salts of the above metals can be obtained, for example, from the pentachlorides of the metals, in the manner described, for example, in U.S. Patent No. 2,951,742 granted September 6, 1960, to Walter Schel-ler, in US. patent application Ser. No. 695,- 213, now Patent 2,974,007, filed November 8, 1957, by Walter Scheller and in US. patent application Ser. No. 13,585, now Patent 3,068,066, filed March 8, 1960, by Walter Scheller et al.

7 The reduced halides, the alkali metal halide double salts and the reduced alkali metal halide double salts diifer favorably irom the pentachlorides in that they have a considerably lower vapor pressure, and owing to their lower sensitivity to air and moisture a lower tendency to form oxychlorides. This considerably simplifies the treatment. The lower vapor pressure has the further advantage that both in the pre-treatment and carrying out the reduction the loss of metal by the evaporation of halide is reduced. Furthermore, secondary reactions are avoided, such as the reduction of the pentachloride by free metal to sub-chlorides (NbCl -l-NbCl The lower tendency for the formation of oxychlorides leads to metal having a considerably reduced oxygen content.

It is of advantage to disintegrate, for example, grind or pulverize, the reduced starting material and to mix it with the magnesium with the exclusion of air, for example, under a protective gas. In view of the lower sensitivity to air and moisture of the starting materials, the apparatus required is considerably simpler.

The reduction of the starting material with magnesium is advantageously carried out in an open vessel. After pouring the mixture into the vessel it is ignited by being locally overheated at the surface so that the thermite reaction proceeds downwardly through the mass. In order to start the ignition local overheating to a temperature to about 450 C. is required. It is of advantage to use a mixture containing a quantity of magnesium in excess of the stoichiometric quantity required for the reduction. It is of advantage to ignite the mixture in several places, instead of one, for example, at a number of places ranged in the form of a ring. In this manner the reaction proceeds uniformly from the upper surface to the bottom of the mixture, and the loss of metal by evaporation or turbulence of the reduction mixture is considerably reduced, or avoided.

The advantages of the process of the invention will be understood by comparing that process with the prob, ess hitherto used for reducing the pentachloride with magnesium. At the temperature required for the reaction the pentachlorides of niobium and tantalum have an extremely high vapor pressure. Thus, the pentachlorides exert a vapor pressure of one atmosphere at 247 C. and .232" C., respectively. These high vapor pressures cause losses in starting material, and consequently of metal, unless the whole apparatus is built to withstand such a high pressure, which complicates the apparatus. Moreover, at those temperatures the pentachlorides are highly corrosive. Losses can also occur at the high temperature due to the mixture being blown 'out of the reaction vessel. An earlier proposal to add alkali metal halides to liquid reaction mixtures aitorded no remedy because a purely mechanical physical mixture of the metal pentachloride and an alkali metal halide does not lower the vapor pressure of the pentachloride.

In contradistinction thereto the vapor pressure of the halides of lower valency and especially of the alkali metal halide double salts is very much lower, so that the disadvantages mentioned above do not occur when these starting materials are used.

The process of the invention can be used for the direct production of alloys of the two metals. For this purpose the halides of the two metals are mixed together in the proportions desired in the alloy, before the halides are reduced with magnesium. The alloy obtained by the reduction is quite homogeneous. An equally good homogeneity can be attained in producing alloys only by mixing the powdered metals together and subsequently sintering the mixture which is a very clostly procedure.

The reaction temperature may vary within certain limits. By suitably selecting the temperature particle size and other surface properties of the metal obtained can be influenced. The addition of a niobium or tantalum halide, for example, the pentachloride, to a reaction mixture which contains an alkali metal halide double salt, raises the reaction temperature.

An addition of a carrier salt, for example, an alkali metal halide, to the reaction mixture lowers the reaction temperature. The reaction temperature can also be lowered by previously reducing the double halide with hydrogen in the manner described above. The course of the react-ion can also be influenced by the construction of the vessel used for the reaction. Thus, it is of advantage to provide the inside of the reaction vessel with a heat-insulating lining of a refractory and inert material, for example, magnesium oxide or aluminum oxide.

An example of such a reaction vessel is shown in the accompanying drawing. The reaction vessel It} is of metal, for example, of stainless steel. The inside of the vessel is provided with a heat-insulating layer by placing one upon another a number of rings 12 of refracting material. The external diameter of the rings is so chosen that they just fit into the vessel 10' without the need of any intermediate layer or means for fixing the rings. The upper part of the vessel 10 is provided with a connecting flange 14 to produce a vacuum in the container or a protective gas to be introduced, before the reduction. The vessel is closed in a gas-tight manner at its upper end by a cover 18, to the underside of which are fixed a plurality of baffle plates 20. After evacuation and introduction of the protective gas, the connection 14 is closed by an expansion vessel 16 of elastic, but sufiiciently pressure-resistant material, for example, polyethylene. The protective lining formed by the rings 12 covers the inner surface of the vessel up to a distance slightly below the level to which the vessel is charged with the reduction mixture. Therefore, above the uppermost ring the mixture comes into contact with the wall of the vessel and can be ignited by heating the wall at this place by means of a ring burner 22.

The inner lining reduces the loss of heat through the vessel Wall and this favors the growth of the particles during the reduction. In this manner there is obtained a product of coarse particle size, of which the bulk density in the case of tantalum is about 4 to grams per ccm., as compared with a bulk density of about 2 to 3 grams per ccm. of a product obtained in a different manner. Furthermore, contact with the metallic wall of the vessel is substantially avoided, and this ensures the production of a product of higher purity, especially with reference to iron, nickel and chromium. This is especially advantageous in working on a large industrial scale where it is hardly feasible to construct the reaction vessel wholly of inert material. A vessel of the kind described above is also considerably less expensive than a vessel lined with a sheet of tantalum or niobium, as in a short time the latter becomes brittle and must be replaced.

The following examples illustrate the invention:

Example I 20.0 grams of finely divided niobium trichloricle were mixed well with a quantity of magnesium powder amounting to 150% of the theoretical quantity, and the mixture was introduced into a quartz tube lined with sheet tantalum and the mixture was made compact by stamping and vibration. The mixture was ignited by local heating at about 500 C. at the upper open end of the tube. When the reaction had ceased, the mixture was cooled, and the powder Was detached from the sheet tantalum, and the magnesium chloride and excess of metallic magnesium were removed by dissolution with hydrochloric acid. The niobium powder was then washed with water and dried in vacuo. There were obtained 8.85 grams of niobium which represents a yield of 95%.

Example 2 16.4 grams of niobium trichloride and 10.7 grams of tantalum pentachloride were mixed with a quantity of finely divided magnesium amounting to 50% more than the theoretical quantity. With the exclusion of air and moisture in an atmosphere of argon. The mixture was charged into a nickel tube and ignited in the manner described in Example 1. The upper end of the vertical tube was closed with a polyethylene bag that had been scavenged with argon. There were obtained 12.5 grams of a homogeneous alloy of niobium and tantalum which represent a yield of 95%.

Example 3 670 grams of a potassium chloride double salt of niobium and tantalum pentachlorides mixed in the ratio 2:8 were mixed with a quantity of magnesium powder amounting to 30% more than the theoretical quantity and the mixture was charged with the exclusion of air into a steel tube having a diameter of 50 mm. and then locally ignited. The reaction was carried out in an atmosphere of argon. The product was worked up in the manner described in Example 1 and there were obtained 248 grams of niobium-tantalum alloy, which represents a yield of Example 4 520 grams of a potassium chloride double salt of niobium pentachloride (potassium hexachloroniobate) were reduced in the manner described in Example 3. The reaction gave 133 grams of niobium powder, which represents 95% of the theoretical yield. The reaction product contained 0.045% Fe, 0.002% Ni, and 0.010% Cr, 0.13% Mg and 0.30% CI. The iron content of the product can be considerably reduced by using purer magnesium. Most of the aforesaid impurities can be removed by sintering the niobium.

Example 5 To a reaction mixture corresponding to that described in Example 4 a quantity of potassium chloride was added equivalent to the double halide. In contradistinction to Examples 1 to 4, in which the reaction progressed spontaneously through the mixture after being locally ignited, this did not occur and it was necessary to heat the whole reaction tube to complete the reaction. The product was Worked up in the manner described above.

Example 6 The reaction vessel used was made of stainless steel and was constructed as shown in the accompanying drawing. It had an internal diameter of 36 cm. and a total height of cm. The inside of the vessel was lined to a height of about 60 cm. with a protective lining of magnesium oxide 3 cm. thick. The bottom of the vessel was fist covered with a layer 24 of granular potassium chloride having a height of 20 cm. Then followed a mixture of 25 kilograms of the double salt of potassium chloride and tantalum pentachloride (TaCl .KCl) in granular form with 4.2 kilograms of magnesium shavings. This quantity of magnesium is 20% more than the stoichiometric quantity required for the reduction. Upon the resulting layer 26 was applied a protective layer 28 of magnesium shavings 10 cm. high, which served to reduce any tantalum pentachloride given oif by dissociation of the double salt. The vessel was charged in an atmosphere of dry argon. Then the opening at the top was closed with a cover of stainless steel, which cover on its under surface se veral bafile plates. The lowest baffle plate consisted of tantalum. The vessel was then evacuated three times in succession to a pressure of about 5 10- mm. of mercury, and then an atmosphere of argon under atmospheric pressure was established within the vessel. After the evacuation the connection 14 was closed with a polyethylene bag.

The mixture was ignited by heating the wall of the vessel to red incandescence by means of an external ring burner at the place above the protective lining where the mixture of double salt and magnesium makes contact with the wall of the vessel. As soon as the wall reached a temperature of about 500 C. the reaction commenced in the interior of the vessel. It advanced slowly and uniformly downwards and was complete Within a period of about three minutes.

The vessel was then allowed to cool in an atmosphere of argon, and the reaction mixture was removed by drilling and worked up in the manner described in Example 1. There were obtained 9.0 kilograms of tantalum powder of coarse particle size, which represents a yield of metal of about 85%. The bulk density of the product was about 4.5 grams per cc. The metal so obtained had only a trifling content of impurities causing embrittlement so that, after being dehydrogenated in vacuo at 900 C.

and after being fused by electron bombardment, it had a Vickers hardness of 57.

What is claimed is:

A process for the manufacture of a metal selected from the group consisting of niobium, tantalum and binary alloys composed of tantalum and niobium, comprising the steps of mixing at least one salt selected from the group consisting of potassium niobium double chloride of the formula KNbCL; and potassium tantalum double chloride of the formula KTaCl in solid, particulate state with magnesium likewise in solid, particulate state, the proportion of the magnesium being in excess of the stoichiometric proportion required for the reduction of the metal chloride, igniting the mixture at a temperature from about 4-50 to 500 C. by local heating on the surface, thereby starting the reduction reaction which advances slowly and uniformly downwards, and separating the metal from the reaction products.

References Cited in the file of this patent UNITED STATES PATENTS 1,415,516 Bridge May 9, 1922 2,744,006 Winter at al. May 1, 1956 2,843,477 Booge July 15, 1958 2,848,320 Keller et al Aug. 19, 1958 2,890,111 Shelton June 9, 1959 2,891,857 Eaton June 23, 1959 FOREIGN PATENTS 486,703 Canada Sept. 23, 1952 791,121 Great Britain Feb. 26, 1958 OTHER REFERENCES 

